Circuits for reducing harmonic currents that are fed back into a d.c. power supply system by a power converter connected to a motor typically include at least one condenser connected in parallel with the power converter, and at least one line reactor connected in series with the condenser. In one such circuit, the harmonic currents fed back into the d.c. power supply system are limited to the required limit levels by suitably dimensioning the condenser and the line reactor. When requirements are high, i.e., when only a small portion of the harmonic currents may be tolerably fed back into the d.c. power supply system, the condenser and the line reactor must be designed with very large dimensions.
British Patent No. 1,411,440 describes a circuit in which at least one LC series resonant circuit having at least one capacitor and at least one inductor is connected in parallel with the condenser. However, such a circuit can only reduce harmonic currents in a preselected frequency range. This circuit cannot reduce harmonic currents above a preselected minimum frequency. A reduction in harmonic currents above a defined minimum frequency could previously be achieved only by having the line reactor and the condenser overdimensioned. The parallel connection of an RC circuit with the intermediate circuit condenser which is possible with this circuit serves exclusively to increase the input impedance for 50 Hz currents.
In addition, German Patent Application No. 42 19 214 describes a circuit that attenuates harmonics and is connected between a power converter and an a.c. power supply system into which power is supplied. The system thus represents the load for the power converter. The circuit includes a duplex reactor which has a primary winding connected to the power converter and a secondary winding connected to the a.c. power supply system and in anti-parallel connection to the primary winding. The harmonic component in the load current is reduced by the duplex reactor and an LC filter tuned to a predefined frequency and connected to a tapping point of the duplex reactor.